test: Add a test for log filter-*

This exercises a few success and failure modes of the log filter-*
commands. log filter-list is not tested because it's purely informational.
I don't think there's a good way to test it except by testing if the output
of the command exactly matches a sample run.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/log/Makefile b/test/log/Makefile
index fdf5295..88bc573 100644
--- a/test/log/Makefile
+++ b/test/log/Makefile
@@ -3,6 +3,7 @@
 # Copyright (c) 2017 Google, Inc
 
 obj-$(CONFIG_LOG_TEST) += log_test.o
+obj-$(CONFIG_CMD_LOG) += log_filter.o
 
 ifdef CONFIG_UT_LOG